1. Field of the Invention
The invention relates to a substrate for out-of-ground cultivation that exhibits satisfactory properties for the cultivation of plants, and a method for its production.
2. Background of the Related Art
It is important, for the growth of the plants, that the substrate contain air and can absorb and hold water or nutritive aqueous solutions.
For out-of-ground cultivation, substrates made of a felt of mineral fibers, such as mineral or glass wool, have been proposed because these substrates exhibit the advantage of being very porous, the fibers generally occupying at most 5% of the total volume of the substrates. They can serve as a support for the roots, and contain a reserve of air and water or nutritive solution; on the other hand, they are light and chemically inert.
It is known that different substrates are necessary for the cultivation of plants and that their construction depends on the state of growth of these plants. Actually, as the development of the plants proceeds, the substrates must have a greater volume and a structure that makes it possible to offer to them the elements they need (water, air, nutritive solutions) and sufficient space for the development of the roots.
Currently, greenhouse out-of-soil cultivation comprises at least two stages. The first corresponds to the growth of the seedlings and requires substrates often called cubes that are not very bulky, the plant root system being not very developed. During the second stage, substrates of small volume are placed on substrates of larger volume, also called "slabs," to allow a more complete development of the roots and a good growth of the plants. The "slab" substrates generally carry several small substrates known as "cubes."
This invention relates more particularly to this second category of substrate (the "slabs") which not only must offer the plants sufficient volume for the development of the roots but also the amount of water and air necessary for their growth, and this for several plants.
The retention of water, or hydroretentivity, of a substrate is an important characteristic for conducting the cultivation of the plants. Moisture conditions can vary depending on various factors such as the type of plants to be cultivated, the climate, the seasons, the stage of development of the plant. Regardless of the moisture conditions desired for a particular type of substrate, it is necessary that the water be absorbed and held to a certain extent by the substrate: it must not drain off immediately, but must remain available for the plant. Actually, water or another solution too firmly bonded to the substrate, or that drains off too quickly, cannot be used by the plant under normal conditions.
It is known that hydroretentivity is connected to the capillary action of the felt constituting the substrate. The capillary action depends on the diameter of the fibers, therefore on their fineness, and on the density of the felt obtained from these fibers.
For a given density, the hydroretentivity of a felt increases with the fineness of the fibers. With felts which contain fibers of the same average diameter, the hydroretentivity increases when the density increases.
Depending on the type of the plants to be cultivated, it can be advantageous to use substrates of different structures in which it is possible to cause the density and the diameter of the fibers to vary.
For example, a substrate exhibiting a high water retention should contain fibers of small diameter and exhibit a high mass.
If, for a good growth, the plant needs water, it also requires air. Substrates of high density, which exhibit good water retention, useful for the plant, have an air reserve which can be insufficient.
To increase the ability to aerate the roots contained in the substrate, it is possible, for example, to form air passages in the substrate by making holes extending into the substrate. It is also possible to decrease the density of the substrate, considering the fact, however, that a decrease in density causes a decrease in water retention.
However, the presence of holes in the substrate or else a low density can reduce the mechanical strength of the substrate. In particular, when a substrate of low density (to increase its air reserve) comprises fibers of small diameter, which makes it possible to increase the phenomenon of capillary action and, consequently, the hydroretentivity of the substrate, its mechanical strength is reduced. When they have been soaked with water, these substrates can sag, and this all the more when the substrate is used as a "slab" carrying several "cubes." The losses in thickness which result cause a reduction of the air reserve and these losses in thickness are all the higher as the substrate has a lower density and fibers of smaller diameter.
An effort has therefore been made to produce substrates for out-of-ground cultivation which exhibit sufficient mechanical strength to undergo minimum sagging during hydration while exhibiting a water retention and an air reserve that are sufficient for a good development of the roots and a satisfactory growth of the plants.